PH-INDUCED CALCIUM TRANSIENTS IN TYPE-II ALVEOLAR EPITHELIAL-CELLS

Although both intracellular pH (pH(i)) and intracellular Ca2+ concentr ation ([Ca2+]i) are highly regulated and have important metabolic effe cts in alveolar epithelial cells, little is known about the interrelat ionship between these two ions in alveolar epithelial cells. The prese nt study examined changes in [pH]i and [Ca2+]i in isolated alveolar ep ithelial cells using the fluorescent dyes SNARF-1 and fura-2. Basal pH (i) values in freshly isolated and cultured alveolar epithelial cells were 7.27 and 7.24, respectively. Resting [Ca2+]i values in freshly is olated cells (53 +/- 5 nM) were lower than those in cultured type II c ells (107 +/- 21 nM). pH(i) increased rapidly after addition of 25 mM NH4Cl in both cultured and freshly isolated cells and then decreased b ack toward baseline over the following 10 min. The rise in pH(i) was a ssociated with a transient increase in [Ca2+]i. Resuspension of cells in an NH4Cl-free solution resulted in rapid intracellular acidificatio n, which recovered over the subsequent 10 min. Removal of sodium or ad dition of 1 mM amiloride to the external solution slowed the rate of r ecovery from intracellular acidification, consistent with the particip ation of Na+-H+ exchanger in this process. In freshly isolated cells, [Ca2+]i increased following acidification and then decreased as the ce lls recovered from an acid load. In cultured cells, [Ca2+]i also incre ased following acidification but then remained elevated over the subse quent 10 min. The recovery of [Ca2+]i toward baseline values in fresh cells following acidification was dependent on the presence of externa l sodium. These data demonstrate that both increases and decreases in pH(i) of alveolar epithelial cells are associated with increases in [C a2+]i and suggest that some of the metabolic effects of altering pH(i) may be secondary to increases in [Ca2+]i. The dependency of [Ca2+]i r ecovery following acidification on external sodium raises the possibil ity that freshly isolated type II cells have Na+-Ca2+ exchangers that contribute to the regulation of [Ca2+]i.